Packaging materials are increasingly being scrutinized as contributors to litter and other sustainability issues. One solution is to make packaging out of compostable materials. A number of biodegradable starch-containing materials have been developed recently.
Starch-containing compositions have been used to provide biodegradable resin compositions useful for manufacture of shaped articles such as rigid sheet, flexible film, or molded articles (see, for example, U.S. Pat. Nos. 5,043,196, 5,314,754, 5,322,866, 5,374,304, and 7,326,743 and WO2008014573, which describe starch compounded into various water soluble polymers). “Thermoplastic starch” compositions using biodegradable polymers as melting aids have been disclosed in U.S. Pat. Nos. 6,235,815 and 6,472,497 and US20030187149. Other starch-containing compositions include those described in U.S. patent application Ser. No. 61/254,951.
There is a sizable market for compostable trash bags, shopping bags and the like that is currently served by blends of starch and compostable polyesters. The predominant compostable water-insoluble polyester is a copolymer of 1,4-butanediol, terephthalic acid and adipic acid (PBAT), all derived from petroleum sources. PBAT polymers are commercially available under the tradename ECOFLEX® from BASF. Thermoplastic starch compositions have been made from PBAT by compounding the polymer with starch, glycerol and water, as taught in U.S. Pat. Nos. 6,348,524, 6,962,950, 7,176,251, and published patent applications US20070241483(A1) and US200838496(A1).
An even more sustainable approach is to derive the compostable polyester copolymer from one or more bio-sourced monomers. This would significantly reduce the environmental footprint (non-renewable energy consumption and greenhouse gas emissions) of the bag.
For example, E. I. du Pont de Nemours and Company (“DuPont”) has recently developed aliphatic-aromatic copolyesters based on bio-sourced 1,3-propanediol (PDO), including copolymers derived from copolymerization of bio-PDO, terephthalic acid (petroleum sourced) and sebacic acid (bio-sourced from castor beans) that comprise over 50% bio-sourced components (See International Application Numbers PCT/US09/67875 and PCT/US09/67863 and US2009/0123767).
While these 1,3-propanediol-terephthalate-sebacic acid compositions closely match the thermal and compostability properties of commercially-available PBAT, they fall short in tensile and mechanical properties and in particular tear strength, a key requirement for films. In addition, blown films made from these polymers exhibit anisotropy in tear strength: the tear resistance in the machine direction is substantially less than in the transverse direction. PBAT films have more balanced tear strength.
Adding additional monomers, such as phthalic anhydride, can improve tear strength (See International Application Numbers PCT/US09/67831, PCT/US09/67838 and PCT/US09/67850). But these copolymers also exhibit anisotropic tear resistance in blown films.
It is desirable to obtain additional compostable compositions that are sourced from biological sources instead of petroleum sources that provide good tensile and mechanical properties for articles made from the compositions. One such mechanical property is tear strength, which is desirable for packaging film applications, including biodegradable trash bags.